An apparatus for closing a mold in an injection-molding operation has a main piston that is connected to a mold-carrying plate and that defines in a main cylinder front and rear main chambers. A so-called rapid-action piston is fixed to and extends as a rod from the rear wall of the main cylinder into a small-diameter bore in the main piston, and this rear wall is formed with a plurality of fill passages connected via respective one-way valves to the sump. A passage extends axially through the rapid-action piston into its cylinder so that fluid under pressure can be fed through this passage into the small-diameter cylinder in the main piston. Obviously fluid at a given pressure will move the main piston much more rapidly when applied to it in the small-diameter rapid-action chamber than in the large rear main chamber, and the rear main chamber can fill rapidly through the several fill passages.
Such an arrangement is disadvantageous in that it requires several fill passages with their own valves to allow sufficient flow into the rear chamber for fast motion of the main piston, and makes rapid-action opening of the mold very complex or impossible. What is more such an arrangement normally requires a battery of expensive hydraulic control valves.